1. Field of the Invention
The present invention relates to improving rotary aircraft maintenance, and more particularly, to systems and methods for evaluating aircraft rotor blade alignment, collectively.
2. Background
Conventional rotary aircraft include a collective controller that allows a pilot to collectively increase or decrease a pitch angle for main rotor blades. This collective controller changes the pitch angle, i.e., blade angle, of all the main rotor blades by a uniform amount at the same time. When the blade angle collectively changes, the total lift derived from the main rotor increases or decreases. In level flight this causes the aircraft to climb or descend, and, when the aircraft is pitched forward, the aircraft will accelerate or decelerate.
Maintenance on various rotary aircraft components is performed throughout the lifespan of the aircraft and results in various adjustments that affect the alignment of individual main rotor blades, e.g., minimizing rotor blade vibrations. These adjustments can include an adjustment to weight, blade angle, blade tabs, pitch rods and rigging of collective control. The total impact of these adjustments can affect the alignment of the rotor blades, which, in turn can result in degraded or unsafe aircraft performance.
In order to monitor the impact of these adjustments and determine if the pitch angle of the rotor blades is within an acceptable alignment tolerance, a special flight test is conducted to determine the rotor speed measured as rotations per minute for the main rotor blades. For example, this special flight test involves an auto-rotative flight, e.g., un-powered flight, while maintaining a steady heading, stabilized at a specified airspeed (e.g., the airspeed slightly above an airspeed for minimum descent) indicated airspeed with the collective controller positioned to adjust each rotor blade to a flat pitch angle. During flight, pilots monitor and measure parameters such as main rotor blade rotations per minute (RPM), outside air temperature, pressure altitude and fuel weight. From these parameters, the aircraft gross weight and air density altitude are calculated. The pilots then determine if the rotor blades are in proper alignment according to a chart with prescribed tolerances according to aircraft gross weight, and by comparing an indicated rotor speed (RPM) to the air density altitude (feet). As can be readily appreciated by those skilled in the art, this special flight test is expensive, time-consuming, complex, depends on ambient air conditions (e.g., turbulence), and pilot skill
Therefore, there is a need for systems and methods which can evaluate a collective condition of aircraft rotor blades (e.g., alignment of aircraft rotor blades), while the aircraft is on the ground, so as to eliminate the need for a special flight test.